In neurology, the Bereitschaftspotential or BP (from German, "readiness potential"), also called the pre-motor potential or readiness potential (RP), is a measure of activity in the motor cortex of the brain leading up to voluntary muscle movement. The BP is a manifestation of cortical contribution to the pre-motor planning of volitional movement. It was first recorded and reported in 1964 by Hans Helmut Kornhuber and Lüder Deecke at the University of Freiburg in Germany. In 1965 the full publication appeared after many control experiments[1].

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In the spring of 1964 Hans Helmut Kornhuber (then docent and chief physician at the department of neurology, head Professor Richard Jung, university hospital Freiburg im Breisgau) and Lüder Deecke (his doctoral student) went for lunch to the 'Gasthaus zum Schwanen' at the foot of the Schlossberg hill. Sitting alone in the guest garden they discussed their frustration with research on the brain as a passive system prevailing worldwide and their desire to investigate self-initiated action[2].

The possibility to do research on electrical brain potentials preceding voluntary movements came with the advent of the 'computer of average transients,' the first, primitive instrument available at that time in the Freiburg laboratory. In the electroencephalogram little is to be seen preceding actions, except of an inconstant diminution of the α- (or μ-) rhythm. The young researchers stored the electroencephalogram and electromyogram of self-initiated movements (fast finger flexions) on tape and analyzed the cerebral potentials preceding movements time-reversed with the start of the movement as the trigger, literally turning the tape over for analysis since they had no reversal playback or programmable computer. A potential preceding human voluntary movement was discovered and published in the same year (Kornhuber & Deecke 1964). After detailed investigation and control experiments such as passive finger movements the Citation Classic with the term Bereitschaftspotential was published[3].

An interesting use of the BP is in Brain Computer Interface (BCI) applications; this potential can be identified from scalp recording (even from single-trial measurements!) and interpreted for various uses – from the control of computer displays to (,I think, eventually,) control of peripheral motor units in spinal cord injuries. For more on this exciting topic:

The BP is ten to hundred times smaller than the α-rhythm of the EEG; only by averaging, relating the electrical potentials to the onset of the movement it becomes apparent. Figure shows the typical slow shifts of the cortical DC potential, called Bereitschaftspotential, preceding volitional, rapid flexions of the right index finger. The vertical line indicates the instant of triggering t = 0 (first activity in the EMG of the agonist muscle). Recording positions are left precentral (L prec, C3), right precentral (R prec, C4), mid-parietal (Pz); these are unipolar recordings with linked ears as reference. The difference between the BP in C3 and in C4 is displayed in the lowest graph (L/R prec). Superimposed are the results of eight experiments as obtained in the same subject (B.L.) on different days.

Note that the BP has two components, the early one (BP1) lasting from about –1.2 to –0.5; the late component (BP2) from 0.5 to shortly before 0 sec [From Deecke et al. 1976] The pre-motion positivity is even smaller, and the motor-potential which starts about fifty to sixty milliseconds before the onset of movement and has its maximum over the contralateral precentral hand area is still smaller. Thus, it takes great care to see these potentials: exact triggering by the real onset of movement is important, which is especially difficult preceding speech movements. Furthermore artefacts due to head-, eye-, lid-, mouth-movements and respiration have to be eliminated before averaging because such artefacts may be of a magnitude which makes it difficult to render them negligible even after hundreds of sweeps (Grözinger et al. 1980). In the case of eye movements eye muscle potentials have to be distinguished from cerebral potentials. In some cases animal experiments were necessary to clarify the origin of potentials such as the R-wave. Therefore, it took many years until some of the other laboratories were able to confirm the details of Kornhuber & Deecke's results. In addition to the finger or eye movements as mentioned above, the BP has been recorded accompanying willful movements of the wrist, arm, shoulder, hip, knee, foot and toes, it was recorded prior to speaking, writing and also swallowing[4].

The Bereitschaftspotential was received with great interest by the scientific community, as reflected by Sir John Eccles's comment: “There is a delightful parallel between these impressively simple experiments and the experiments of Galileo Galilei who investigated the laws of motion of the universe with metal balls on an inclined plane”[5]. The interest was even greater in psychology and philosophy because volition is traditionally associated with human freedom (cf. Kornhuber 1984). The spirit of the time, however, was hostile to freedom in those years; it was believed that freedom is an illusion. The tradition of behaviourism and Freudism was deterministic. While will and volition were frequently leading concepts in psychological research papers before and after the first world war and even during the second war, after the end of the second world war this declined, and by the mid-sixties these key words completely disappeared and were abolished in the thesaurus of the American Psychological Association[6]. The BP is an electrical sign of participation of the supplementary motor area (SMA) prior to volitional movement, which starts activity prior to the primary motor area[7]. The BP has preciptated a worldwide discussion about free will (cf. the recent book "The Bereitschaftspotential",[8][9].

The BP method has been extensively employed. In studies using conscious experience by means of introspection and the BP, a temporal order of events has been worked out[10] in that the BP started about 0.35 sec earlier than the subject's conscious awareness that 'now he or she feels the desire to make a movement.' Libet concludes that we have no free will in the initiation of our movements, while in movement control we have, since subjects in his experiments were able to successfully pose a last minute 'veto' to the intended movement, i.e. Libet links will totally to consciousness. However, it has to be taken into account that in our brain we have conscious and unconscious matters and both are important. Furthermore, consciousness always has a delay in both the sensory system (e.g. awareness of pain) and in the motor system. In experiments like this, the question of free will cannot be tackled. The free will question has been answered already before the experiment, when the subject has agreed to the experimenter's general instruction. Thus, the free will question is solved once and not with each of the 200 or so single movements necessary for averaging the BP. During the experiment, subjects are in an experimental 'set' making the movements pretty automatically. The frontal cortex that is competent for volition and will, delegates the execution of the movements to other brain structures, such as basal ganglia and primary motor cortex (MI). The supplementary motor area (SMA) as the generator for the early BP (BP1) belongs to the frontal lobe. According to Libet's experiments, BP1 is not yet accompanied by consciousness. When the information has travelled from the SMA via the basal ganglia (motor loop) to the primary motor cortex MI (late BP, BP2), consciousness obviously is switched on in order to make changes and adaptations to the instantaneous situation with the extreme of vetoing it entirely, the most important reason for needing consciousness is to learn from the movement (see below 'further reading').